Pressure transmitters are devices which are utilized in industrial process control systems in order to detect and/or measure the pressure of a monitored process fluid. Such pressure transmitters can perform differential or absolute pressure measurements and therefore are manufactured according to different layouts and models. Most common pressure transmitters are those indicated as gauge or absolute pressure transmitters and differential pressure transmitters.
In many applications, the use of pressure transmitters is particularly advantageous since from one or more measurements of relative, differential or absolute pressure, it is possible to indirectly obtain values that are indicative of other physical variables of the fluid controlled, where such values would be more difficult to be detected directly.
According to a known configuration, a pressure transmitter of includes a main hollow body, sometime referred to as a module housing or sensor housing body, which is suitably shaped to house components carrying out the transduction. This main body includes a measurement chamber housing a pressure sensor. Suitable primary electric/electronic circuits for processing signals arriving from the pressure sensor may also be housed into the main hollow body.
A transmitter body is coupled to the sensor housing body and contains further components, such as, for example, displays for locally displaying the values measured, secondary electronic circuits for processing the signals arriving from the pressure sensor and for communicating with other transmitters or with remote control units, etc.
In order to perform the required sensing and measurement operations, the pressure transmitter includes a further part or body which must be placed in contact with the process fluid. For this purpose, the additional part is provided with one of more isolation diaphragms which are in fluid communication with the pressure sensor and are suitable to separate the process fluid from the circuit inside the transmitter. At least one of the isolation diaphragms is positioned on this additional part so as to have an external surface exposed directly to the process fluid under monitoring.
This additional part can be a separate body connected to the sensor housing body, for example, by screwing or welding, or it can be realized monolithically with the sensor housing body.
At the current state of the art, although known pressure transmitters can adequately perform the tasks they are required to execute, there is still room for further improvements of their structure and functioning.
For example, some possible drawbacks of known pressure transmitters may result from the way the isolation diaphragms, and more specifically the isolation diaphragms which directly interface with the process fluid under control, are positioned on the body supporting them, especially when pressure transmitters are used in very aggressive environments.
For instance, an isolation diaphragm is usually constituted by a thin metallic membrane which is suitably welded onto its supporting body. When the pressure transmitter is intended for being used in special applications (e.g., environments with hot temperatures, and/or abrasive or corrosive process fluids and so on), the isolation diaphragm is made of special materials, such as nickel alloys.
When this thin membrane made of special material is welded on the supporting body which is usually made of a common metallic material such as a stainless steel, the thin membrane may partially melt and mix with the stainless steel of the supporting body.
Hence, such a welding process, in addition to being quite difficult, can result in a welding seam which is to some extent defective. Furthermore, the mechanical characteristics of the isolation diaphragm are deteriorated, and the isolation diaphragm itself may have one or more points of inception of corrosion, which in some cases results in the diaphragm having to be discarded.